Spread-spectrum signal receiver apparatus and interference cancellation apparatus

ABSTRACT

In a spread-spectrum signal receiver apparatus for receiving a spread-spectrum signal and demodulating transmit data from the signal, a receiver receives a spread-spectrum signal that has been spread by a spreading code comprising a combination of a first code that varies depending upon spreading factor and a second code that differs for every user, an interference canceller produces a replica of an interference signal from the receive signal using a despreading code comprising a combination of the first code, which is regarded as a code decided based upon a minimum spreading factor, and the second code that differs for every user, and a receive modulator demodulates transmit data from a signal obtained by subtracting the replica from the receive signal.

BACKGROUND OF THE INVENTION

This invention relates to an spread-spectrum signal receiver apparatusand to an interference cancellation apparatus. More particularly, theinvention relates to a spread-spectrum signal receiver apparatus forreceiving a spread-spectrum signal, which has been spread by a spreadingcode comprising a combination of a first code decided by a spreadingfactor and a second code that differs for every user, and demodulatingtransmit data from the received signal, and to an interferencecancellation apparatus for generating a replica of an interferencesignal from the received signal.

In spread-spectrum communications, W-CDMA (Wideband Code DivisionMultiple Access), which employs direct-sequence spreading, is one of thethird-generation mobile communications systems the standardization ofwhich is being forwarded by the 3GPP.

With CDMA, as shown in FIG. 10, a mobile station, which is aspread-spectrum signal transceiver, has a first modulator la forapplying BPSK modulation (see FIG. 11) to control data that includespilot data, and a first spreader 1 b for applying spread-spectrummodulation using a spreading code for the control data. The mobilestation further includes an encoding circuit 1 c for subjecting thetransmit data (user data) to suitable encoding such as convolutionalcoding, a second modulator 1 d for subsequently applying BSPK modulationand a second spreader 1 e for spreading the resultant signal using aspreading code for the user data. The mobile station further includes amultiplexer 1 f for mapping the control data and user data, which havebeen spread by the first and second spreaders, as an I-axis component(I-channel component) and Q-axis component (Q-channel component) of anI-Q complex plane, as illustrated in FIG. 11, and multiplexing theresulting signals, and a radio transmitter unit 1 g for subjecting themultiplexed signal to frequency conversion and high-frequencyamplification and transmitting the resulting signal from an antenna 1 h.It should be noted that the I and Q channels are referred to also asdata and control channels, respectively. The spreading codes used in thefirst and second spreaders 1 a, 1 e are obtained by multiplying a useridentification code (long code) and a channel identification code (shortcode), which is for identifying the data channel or control channel.

An uplink signal from the mobile station to a base station has a frameformat shown in FIG. 12. One frame has a duration of 10 ms and iscomposed of 15 slots S₀ to S₁₄. User data is mapped to the I channel(data channel) and control data, which is data other than the user data,is mapped to the Q channel (control channel). Each of the slots S₀ toS₁₄ of the data channel that transmits the user data is composed of nbits, where n varies depending upon the transmission rate. Thetransmission rate will be 7.5 (=5×15/10×10⁻³) kbps if n=5 holds and 30kbps if n=20 holds.

Each slot of the control channel that transmits the control data iscomposed of 10 bits, and the transmission rate is a constant 15 kbps.Each slot transmits a pilot, transmission-power control data TPC, atransport format combination indicator TFCI and feedback informationFBI. The pilot is utilized on the receive side for synchronous detectionand SIR measurement, the TPC is utilized for control of transmissionpower, the TFCI transmits the transmission rate of the data and thenumber of bits per frame, etc., and the FBI is for controllingtransmission diversity at the base station. It should be noted that thedata transmission rate and the spreading factor have a 1:1 relationship,and that the spreading factor of the data channel is found from thetransmission rate.

Thus, there are instances where the transmission rates on the data andcontrol channels differ. In such case the spreading factor [=(symbolperiod)/(chip period)] on the data channel differs from that on thecontrol channel. For example, (1) if the transmission rate of the datachannel is lower than that (15 kbps) of the control channel, then thespreading factor of the data channel will be larger than that of thecontrol channel, and (2) if the transmission rate of the data channel ishigher than that (15 kbps) of the control channel, then the spreadingfactor of the data channel will be smaller than that of the controlchannel. The larger the spreading factor, the higher the process gain.Accordingly, in a W-CDMA system, transmission power for which thespreading factor is larger is reduced to lower the total transmissionpower. In other words, with W-CDMA, the control and data channels aresubjected to BPSK modulation to effect spread-spectrum modulation atpowers that differ from each other, the spread-spectrum modulatedsignals are mapped on an I-Q complex plane and multiplexed and themultiplexed signal is transmitted.

If, by way of example, the spreading factor of the data channel islarger than that of the control channel, then, as shown in FIG. 13, theapparatus of FIG. 10 is further provided with multipliers 1 h, 1 i, themultiplier 1 h multiplies the BPSK modulation output of the secondmodulator id of the data channel by βc (βc<1) and the multiplier 1 imultiplies the BPSK modulation output of the first modulator 1 a of thecontrol channel by 1 (i.e., leaves this output unchanged). The first andsecond spreaders 1 b, 1 e thenceforth spread-spectrum modulate theoutputs of the multipliers 1 i, 1 h, respectively, the multiplexer 1 fmaps the spread-spectrum modulated signals of the respective channels onthe I-Q complex plane, as illustrated in FIG. 14A, and multiplexes theresultant signals, and the radio transmitter unit 1 g subjects themultiplexed signal to a frequency conversion and high-frequencyamplification and transmits the resulting signal from the antenna 1 h.By thus lowering the transmission power of the channel having the largerspreading factor, the total transmission power can be controlled(reduced).

Further, if the spreading factor of the data channel is made smallerthan that of the control channel, the multiplier 1 h multiplies the BPSKmodulation output of the second modulator 1 d by 1 and the multiplier 1i multiplies the BPSK modulation output of the first modulator 1 a by β.The multiplexer 1 f maps the spread-spectrum modulated signals of therespective channels on the I-Q complex plane, as illustrated in FIG.14B, and multiplexes the resulting signals. As a result, the totaltransmission power can be reduced by lowering the transmission power ofthe channel having the larger spreading factor.

FIG. 15 is a block diagram illustrating one channel of the receiversection of a base station. The base station has a radio unit 2 a forfrequency-converting a high-frequency signal received from an antennaATN to a baseband signal; a quadrature demodulator 2 b for subjectingthe baseband signal to quadrature detection, converting the analogin-phase component (I component) and analog quadrature component (Qcomponent) to digital data and distributing the data to a searcher 2 cand fingers 2 d ₁˜2 d _(n). Upon receiving input of a direct-sequencesignal (DS signal) that has been influenced by the multipath effect, thesearcher 2 c detects multipath interference by performing anautocorrelation operation using a matched filter and inputsdespreading-start timing data and delay-time adjustment data of eachpath to the fingers 2 d ₁˜2 d _(n). A control-channel despreader 3 a ofeach of the fingers 2 d ₁˜2 d _(n) subjects a direct wave or delayedwave that arrives via a prescribed path to despread processing using acode identical with the spreading code for the control channel,integrates the results of despreading, then applies delay processingthat conforms to the path and outputs a control-data signal. Adata-channel despreader 3 b subjects a direct wave or delayed wave thatarrives via a prescribed path to despread processing using a codeidentical with the spreading code for the data channel, integrates theresults of despreading, then applies delay processing that conforms tothe path and outputs a user-data signal.

A channel estimation unit 3 c estimates the fading characteristic of thecommunication path using the pilot signal contained in the despreadcontrol-data signal, executes channel estimation which compensates forthe effects of fading, and outputs a channel estimation signal. Channelcompensation units 3 d, 3 e multiply the despread control-data signaland despread user-data signal by the complex-conjugate signal of thechannel estimation signal to thereby compensate for fading.

A RAKE combiner 2 e combines and outputs the control-data signals outputfrom the fingers 2 d ₁˜2 d _(n). A decoder 2 g applies error-correctiondecoding processing to the data that is output from the RAKE combiner 2e, decodes the control data that prevailed prior to encoding and outputsthe decoded data. A RAKE combiner 2 i of the data channel combines andoutputs the user-data signals output from the fingers 2 d ₁˜2 d _(n),and a decoder 2 m applies error-correction decoding processing to thedata that is output from the RAKE combiner 2 i, decodes the user datathat prevailed prior to encoding and outputs the decoded data.

Thus, with the CDMA scheme, a prescribed code is assigned to a user andmultiple users communicate simultaneously. However, because signals fromother channels currently engaged in calls constitute interference, thenumber of channels (users) that can communicate simultaneously islimited. Interference suppression techniques such as interferencecancellers and adaptive array antennas are effective in increasingchannel capacity and research relating to these techniques isprogressing.

If we let Tc represent the period (chip period) of a spreading code andlet T represent the symbol period of a narrow-band modulated signal thatundergoes modulation by the spreading code, then T/Tc will be thespreading factor. By applying spread-spectrum modulation to anarrow-band modulated signal NM, as shown in (A) of FIG. 16, thebandwidth is enlarged by a factor of T/Tc, as indicated by SM, as aresult of which the energy is spread. As a consequence, ifspread-spectrum modulated signals are emitted from the mobile stationsof multiple users simultaneously, the signals overlap one another in themanner shown in (B) of FIG. 16. If a signal from one user, e.g., user 1,is demodulated from these overlapping signals by despreading, the resultwill be as shown in (C) of FIG. 16. The spread signals of users 2 and 3constitute interference signals with respect to the narrow-band signalof user 1. The spectrum ratio of the narrow-band signal of user 1 to theinterference signal is referred to as the Signal Interference Ratio(SIR). The larger the number of users, the smaller the SIR. This meansthat there is a limit upon the number of channels that can communicatesimultaneously (i.e., that there is a limit upon channel capacity). Aninterference canceller seeks to enlarge the SIR and thereby increasechannel capacity, or to reduce transmission power, by suppressing thespread signals of other users, as depicted in (D) of FIG. 16.Specifically, an interference canceller suppresses interference bygenerating a replica of an interference signal using the results ofdemodulating each of the receive signals and subtracting the replicafrom the receive signal.

FIG. 17 is block diagram illustrating a CDMA receiver of a base stationhaving an interference canceller. Specifically, the receiver includes aninterference canceller 101 and receive-signal demodulators 102 a to 102k, which are for users 1 to k, respectively, provided for respectiveones of receive channels. The interference canceller 101 is providedwith interference cancellation units (ICU) 111 ₁˜111 _(k) correspondingto respective ones of the receive channels. The interferencecancellation units 111 ₁˜111 _(k) generate interference replicas of chiprates from the receive signal and output the replicas. Morespecifically, each of the interference cancellation units 111 ₁˜111 _(k)multiplies the receive signal by a dispreading code, then discriminatesdata using the despread signal, lastly spreads the discriminated dataagain, thereby generating the interference replica. A combiner 112combines the interference replica signals of the respective receivechannels, a filter 113 limits the band of the combined interferencereplica signals, a delay unit 114 delays the receive signal for a lengthof time required for generation of an interference replica, and asubtractor 115 executes interference suppression by subtracting thecombined interference replica from the receive signal. The interferencecancellation units produce replicates (replicates of control data andreplicates of user data) of the transmit signal having the symbol rate.These replicates are referred to as symbol replicas and are transmittedto the receive demodulator after interference is eliminated. As aresult, not only is interference from other channels eliminated but sois interference from the multipath effect of the channel in question.The interference cancellation units 111 ₁˜111 _(k) are connected inparallel and shorten processing time by processing all channelssimultaneously.

FIG. 18 is a diagram showing the structure of each of the interferencecancellation units 111 ₁˜111 _(k) according to the prior art. Eachinterference cancellation unit includes a despreader 151 for multiplyingthe receive signal by a despreading code that is identical with thespreading code, thereby outputting a despread signal; a demodulator 152for demodulating “1”, “0” of user data and control data on the basis ofthe result of despreading; an attenuator 153 for attenuating thedemodulated signal by multiplying the result of demodulation by adamping coefficient that conforms to the degree of reliability; are-spreader 154 for spreading the demodulated signal again to therebyoutput an interference replica; a despread-information extraction unit155 for identifying the spreading factor on the transmit side bycollecting TFCI bits, which are contained in the control data, over theduration of one frame; and a symbol-replica interface 156 for creatingand sending a symbol replica.

The despreader 151 has fingers 151 ₁ to 151 _(n). A searcher (not shown)detects multipath and inputs despread-start timing data and delay-timeadjustment data of each path to the fingers 151 ₁ to 151 _(n). Each ofthe fingers 151 ₁ to 151 _(n), has a despread unit for a control channelDPCCH for subjecting the receive signal to despread processing using acode identical with the spreading code of the control channel,integrating the result of despreading, subsequently subjecting theresulting signal to delay processing that conforms to the path andoutputting a control-data signal; and a despread unit for a data channelDPDCH for subjecting the receive signal to despread processing using acode identical with the spreading code of the data channel, integratingthe result of despreading, subsequently subjecting the resulting signalto delay processing that conforms to the path and outputting a user datasignal.

A channel-estimation/AFC circuit 151 b estimates the fadingcharacteristic of the communication path using the pilot signalcontained in the despread control-data signal output from a selector 151g, executes channel estimation in order to compensate for the effects offading, and outputs a channel estimation signal. Channel compensationunits 151 c, 151 d multiply the despread control-data signal anddespread user-data signal by the complex-conjugate signal of the channelestimation signal to thereby compensate for fading. RAKE combiners 151e, 151 f combine the despread signals, from which fading has beeneliminated, output from the fingers and output the results todemodulators 152 a, 152 b, respectively. The demodulators 152 a, 152 bdiscriminate “1”, “0” of the user data and control data based upon thesignals output from the RAKE combiners 151 e, 151 f. Since the pilotsignal is already known, a selector 153 a outputs the control data uponreplacing the demodulated pilot signal with the known pilot signal.

The attenuator 153 has multipliers 153 b, 153 c for multiplying thedemodulated user data and control data by a first damping coefficient athat conforms to the degree of reliability, and multipliers 153 d, 153 efor multiplying the user data and control data by a second dampingcoefficient β that conforms to the degree of reliability, therebyapplying damping. The damping coefficients α, β are set in advance basedupon transmission power, the interference environment, etc., by way ofexample.

The symbol-replica interface 156 multiplies the output signals of themultipliers 153 b, 153 c by the channel estimation signal (complexsignal) that is output from the channel-estimation/AFC circuit 151 b,thereby adding on the fading characteristic of the transmission path,and sends the results of multiplication to the corresponding one of thereceive demodulators 102 a to 102 k (see FIG. 17) as symbol-replicasignals.

Multipliers 154 a, 154 b of each of the fingers 154 ₁ to 154 _(n) of there-spreader 154 multiply the user data and control data output from theattenuator 153 by the channel estimation signal (complex signal),thereby adding on the fading characteristic of the transmission path.Re-spread units 154 c, 154 d spread the user data and control data, ontowhich fading has been added, by a code identical with the despreadingcode of the despreader 151, and outputs the spread signals. An adder 154e combines the spread signals, which are output from the respectivefingers, by data channel and by control channel, thereby generatinginterference replicas that are input to the receive demodulators 102 ato 102 k of the succeeding stage.

The despread-information extraction unit 155 identifies the spreadingfactor on the transmit side by collecting TFCI bits, which are containedin the control data, over the duration of one frame and inputs thespreading factor to the despreading unit 151 a. The latter decides thespreading code of the data channel based upon the spreading factor andperforms despreading using this spreading code.

As shown in FIG. 19, the spreading code on the transmit side is theresult of multiplying the user identification code (scramble code) SCi,which is for identifying the user, by a channel identification code CCi,which is for identifying the channel (data channel or control channel).The user identification code SCi is not dependent upon the spreadingfactor SF but the channel identification code CCi of the data channel isdependent upon the spreading factor SF and varies depending upon thespreading factor. It should be noted that the channel identificationcode of the control channel is fixed because the spreading factor isfixed. FIG. 20A shows a code tree useful in describing the relationshipbetween spreading factor and channel identification code of the datachannel, and FIG. 20B is a diagram useful in describing the relationshipbetween channel identification codes of data channels for which SF=2^(n)and SF=2^(n+1) hold. Here a 1 in the brackets signifies “0” and a −1signifies “1”. Further, a channel identification code is expressed byC_(ch,SF,k), where the suffix SF indicates the spreading factor and thesuffix k the code number. If SF=4 holds, then four 4-bit channelidentification codes C_(ch,4,0) to C_(ch,4,3) exist on the basis of FIG.20A; if SF=8 holds, then eight 8-bit channel identification codesC_(ch,8,0) to C_(ch,8,7) exist. Channel identification codes similarlyexist for other spreading factors.

In a case where there is only one data channel, the channelidentification code of the data channel is C_(ch,SF,k) (where k=SF/4holds). If the spreading factor SF is equal to 4, therefore, then thechannel identification code of the data channel will be C_(ch,4,1) (1,1, −1, −1); if SF=8 holds, the channel identification code of the datachannel will be C_(ch,8,2) (1, 1, −1, −1, 1, 1, −1, −1); if SF=16 holds,the channel identification code of the data channel will be C_(ch,16,4)(1, 1, −1, −1, 1, 1, −1, −1, 1, 1, −1, −1, 1, 1, −1, −1). Thus, thechannel identification code of the data channel varies depending uponthe spreading factor SF. FIGS. 21(a) and (b) are diagrams useful indescribing the relationship between data and channel identification codein a case where the spreading factor SF is equal to 16 [FIG. 21(a)] anda case where the spreading factor SF is equal to 4 [FIG. 21(b)].

Thus, since the spreading-factor information is included in the TFCIbits, the despread-information extraction unit 155 (FIG. 18) collectsthe TFCI bits over the duration of one frame to identify the spreadingfactor on the transmit side.

The transmission rate of the control channel is fixed at 15 kbps and thespreading factor of the control channel is fixed. The channelidentification code of the control channel therefore is fixed, asmentioned above. For example, the channel identification code of thecontrol channel is fixed and is C_(ch,246,0). If the user identificationcode is decided at the time of a call, therefore, the spreading code ofthe control channel will be evident. However, the transmission rate ofthe data channel is variable, the spreading factor varies in dependenceupon the transmission rate and the channel identification code varies,as mentioned above. Consequently, even though the user identificationcode is decided at the time of a call, the spreading code of the datachannel is unknown until the spreading factor is ascertained.

Accordingly, first the despreading unit 151 a despreads only the controlchannel, finds the spreading factor from the TFCI bits to decide thespreading code of the data channel and then starts despreading the datachannel.

FIG. 22 is a diagram showing the structure of each of the receivedemodulators 102 a to 102 k. Each of these demodulators includes fingers121 ₁ to 121 _(n), RAKE combiners 122 a, 122 b of the data and controlchannels, respectively, decoders 123 a, 123 b of the data and controlchannels, respectively, and a despread-information extraction unit 124for identifying and outputting the spreading factor on the transmitside. Each of the fingers 121 ₁ to 121 _(n) has a despreader 131 foroutputting a despread signal of a signal (interference cancelleroutput), which has been obtained by eliminating an interference signalfrom the receive signal, in sync with path timing that enters from asearcher (not shown). More specifically, the despreader of the controlchannel subjects the output signal of the interference canceller todespread processing using a code identical with the spreading code forthe control channel, integrates the result of despreading, subsequentlysubjects the resulting signal to delay processing that conforms to thepath and outputs the processed signal. Further, the despreader of thedata channel subjects the output signal of the interference canceller todespread processing using a code identical with the spreading code forthe data channel, integrates the result of despreading, subsequentlysubjects the resulting signal to delay processing that conforms to thepath and outputs the processed signal.

Combiners 132 a, 132 b generate transmit signals on the transmit side byadding the symbol replicas of the data and control channels DPDCH, DPCCHto the despread signals of the data and control channels, respectively.A channel-estimation/AFC circuit 133 estimates the fading characteristicof the communication path based upon a pilot signal that enters from aselector 135, and channel correction units 134 a, 134 b apply channelcorrection processing to the signals output from the combiners 132 a,132 b, respectively, using the respective channel estimation signals,thereby eliminating fading. The RAKE combiners 122 a, 122 b combine thedata signals and control signals, respectively, output from therespective fingers and from which fading has been eliminated, and inputthe combined signals to the decoders 123 a, 123 b, respectively. Thedecoders 123 a, 123 b apply error correction processing to the user-datasignal and control-data signal output from the RAKE combiners 122 a, 122b, decode the user data and control data that prevailed prior toencoding and output the decoded data.

The despread-information extraction unit 124 identifies the spreadingfactor on the transmit side by collecting TFCI bits, which are containedin the control data, over the duration of one frame and inputs thespreading factor to the despreading unit 131. The latter decides thespreading code of the data channel based upon the spreading factor.Furthermore, the despreading unit 131 first despreads only the controlchannel, finds the spreading factor from the TFCI bits to decide thespreading code of the data channel and then starts despreading the datachannel.

Thus, in accordance with an interference canceller, interference can besuppressed by generating the replica of an interference signal andsubtracting the replica from the receive signal. This makes it possibleto enlarge channel capacity or to reduce transmission power.

The conventional interference canceller is such that if notification ofthe spreading factor is given in advance, delay can be made severalsymbols or less. However, in a communication environment in which theamount of data, i.e., the data speed, changes from time to time, as inthe case of packet communication, the spreading factor varies from frameto frame or from slot to slot. In such case the spreading factor of thedata channel will not be clarified and the channel identification codewill not be determined unless the control data (TFCI bits) in one frameor one slot is demodulated. In other words, the conventionalinterference canceller cannot execute despread processing with regard tothe data channel until the channel identification code is clarified.This means that a processing delay in frame or slot units occurs.Consequently, when transmission power control is carried out using TPC(Transmission Power Control), there tends to be an increase in controlloop delay and the capacity characteristic deteriorates.

FIG. 23 shows an example of the general construction of the despreadingunit 151 a in a case where the spreading factor of the data channelDPDCH cannot be identified unless the control information on the controlchannel DPCCH is demodulated. The despreading unit 151 a includes amultiplier circuit 151 a-1 for spreading the receive signal by aspreading code C_(DPCCH) for the control channel, a multiplier circuit151 a-2 for spreading the receive signal by a spreading code C_(DPDCH)for the data channel, a delay circuit 151 a-3 for delaying the receivesignal, and a spreading-code generator 151 a-4 for generating thespreading codes C_(DPCCH), C_(DPDCH) for the control and data channels,respectively.

In the case of the data channel DPDCH, it is necessary to delay thereceive signal until the spreading factor is clarified by the TFCI bitand the spreading code C_(DPDCH) for the data channel is determined. Thedelay circuit 151 a-3 is provided for this purpose. Since this delay isa frame or slot delay (the delay differs depending upon the units inwhich the control information is multiplexed), the delay has a majoreffect upon TPC (Transmission Power Control).

FIG. 24 is a time chart associated with the block diagram shown in FIG.23. Here SFi signifies the spreading factor. The spreading factor of thecontrol channel DPCCH is SF1 and is fixed, whereas the spreading factorof the data channel DPDCH takes on various values. In FIG. 24, thecontrol information is multiplexed in frame units. In the interferencecanceller, the receive signal is delayed by one frame or more until thespreading factor SF is identified and despreading of the data channel isbegun. Thereafter, in order to produce an interference replica, a largedelay of one frame or more occurs to start interference removal.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to shorten delay timein an interference canceller to a delay time on the order of symbolunits.

Another object of the present invention is to shorten delay in theinterference canceller of a CDMA system that uses a communication formatof the type in which the spreading factor cannot be identified unlessthe control information of the frame or slot is demodulated.

Another object of the present invention is to prevent erroneousinterference removal by so arranging it that removal of interference isnot carried out in a case where data on the data channel DPDCH is notbeing received.

A further object of the present invention is to so arrange it that aprescribed coefficient βc can be reflected in an interference replica ina case where the coefficient βc, which conforms to the spreading factorof the data channel DPDCH and the spreading factor of the controlchannel DPCCH on the transmit side, is used to multiply the signal onone of the channels and the resulting signal is transmitted.

A further object of the present invention is to so arrange it that ahighly precise interference replica is generated by estimating thespreading factor SF from the power ratio between the data channel DPDCHand the control channel DPCCH using a spreading-factor estimation unit,and producing an interference replica using the spreading factor havingthe higher degree of certainty.

According to a first aspect of the present invention, the foregoingobjects are attained by providing a spread-spectrum signal receiverapparatus for receiving a spread-spectrum signal and demodulatingtransmit data from the signal, comprising: (1) a receive unit forreceiving a spread-spectrum signal that has been spread by a spreadingcode comprising a combination of a first code that varies depending uponspreading factor and a second code that differs for every user; (2) aninterference canceller for producing a replica of an interference signalfrom the receive signal using a despreading code comprising acombination of the first code, which is regarded as a code decided basedupon a minimum spreading factor, and the second code that differs forevery user, and generating a signal obtained by subtracting the replicafrom the receive signal; and (3) a demodulator for demodulating transmitdata, from the signal from which the replica has been subtracted, bydespread processing using a spreading code on the transmit side.

The first code (channel identification code) decided based upon thespreading factor is obtained by systematically repeating a code thatconforms to a minimum spreading factor SF_(min). In the presentinvention, therefore, the first code is regarded as a code decided basedupon the minimum spreading factor and despreading necessary forcancellation of interference is performed using a despreading codecomprising a combination of the first code and the second code thatdiffers from user to user. If this arrangement is adopted, it will notbe necessary for the interference canceller to decide the spreading codeafter identifying the spreading factor from the control information(TFCI bits). This makes it possible to shorten delay time tillgeneration of the interference replica.

According to a second aspect of the present invention, the foregoingobjects are attained by providing an interference cancellation apparatusfor receiving a spread-spectrum signal that has been spread by aspreading code comprising a combination of a first code that variesdepending upon spreading factor and a second code that differs for everyuser, and generating a replica of an interference signal from thereceive signal, the apparatus having a replica producing unit forproducing a replica of the interference signal from the receive signalusing a despreading code comprising a combination of the first code,which is regarded as a code decided based upon a minimum spreadingfactor, and the second code that differs for every user. The replicaproducing unit includes: (1) a despreader for despreading the receivesignal using the despreading code comprising the combination of thefirst code, which is regarded as a code decided based upon a minimumspreading factor, and the second code that differs for every user; (2) ademodulator for demodulating transmit data from the despread signal; (3)an attenuator for multiplying the demodulated transmit data by aprescribed damping coefficient; and (4) a spreader for generating thereplica by spreading the attenuated transmit data using a code identicalwith the despreading code.

In accordance with this interference cancellation apparatus, it isunnecessary to decide the spreading code after the spreading factor isidentified from the control information (TFCI bits). This makes itpossible to shorten delay time till generation of the interferencereplica.

Further, the replica producing unit is provided with adamping-coefficient altering unit for detecting that user data is notbeing transmitted on a data channel and making the damping coefficient,which conforms to the data channel DPDCH, equal to zero. If thisexpedient is adopted, removal of interference will not be carried out ifdata on the data channel DPDCH is not being received. This makes itpossible to prevent erroneous interference cancellation.

Further, the replica producing unit is provided with adamping-coefficient altering unit for altering the damping coefficientof the data channel based upon the ratio of receive power on the datachannel to receive power on the control channel. As a result, in a casewhere a coefficient βc in accordance with the spreading factor of thedata channel DPDCH and the spreading factor of the control channel DPCCHon the transmit side is used to multiply the signal on one of thechannels and the resulting signal is transmitted, the coefficient βc canbe reflected in the interference replica.

Another replica producing unit according to the present inventionincludes: (1) a first despreader for despreading a receive signal usinga despreading code comprising the combination of the first code, whichis regarded as a code decided based upon a minimum spreading factor, andthe second code that differs for every user; (2) a spreading-factorestimation unit for estimating the spreading factor SF on the transmitside; (3) a second despreader for generating a despread signal of thereceive signal by integrating, m times, the result of despreading, whichis output from the first despreader, based upon the despreading codeconforming to the minimum spreading factor, where m (an integer)represents the ratio of the estimated spreading factor to the minimumspreading factor; (4) a demodulator for demodulating the transmit datafrom the despread signal; (5) an attenuator for multiplying thedemodulated transmit data by a prescribed damping coefficient; and (6) aspreader for generating the replica by spreading the attenuated transmitdata using a code identical with the despreading code. In accordancewith this replica producing unit, the spreading factor SF can beestimated by a spreading-factor estimation unit from the power ratiobetween the data channel DPDCH and control channel DPCCH, and a highlyprecise interference replica can be generated by producing aninterference replica using the more reliable spreading factor.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the structure of an interferencecancellation unit according to a first embodiment of the presentinvention;

FIG. 2 is a diagram useful in describing the timings of a receive signaland interference replica according to the present invention;

FIG. 3 is a diagram showing the structure of a receive demodulatoraccording to the present invention;

FIG. 4 is a diagram showing the structure of an interferencecancellation unit according to a second embodiment of the presentinvention;

FIG. 5 is a diagram showing the structure of an interferencecancellation unit according to a third embodiment of the presentinvention;

FIG. 6 is a diagram showing the structure of an interferencecancellation unit according to a fourth embodiment of the presentinvention;

FIG. 7 is a diagram showing the structure of an interferencecancellation unit according to a fifth embodiment of the presentinvention;

FIG. 8 is a diagram showing the structure of an interferencecancellation unit according to a sixth embodiment of the presentinvention;

FIG. 9 is a diagram showing the structure of an interferencecancellation unit according to a seventh embodiment of the presentinvention;

FIG. 10 is a block diagram illustrating the structure of a CDMAtransmitter in a mobile station;

FIG. 11 is a diagram useful in describing BPSK modulation and mapping toan I-Q complex plane according to the prior art;

FIG. 12 illustrates the frame format of an uplink frame according to theprior art;

FIG. 13 is a block diagram of a mobile station in W-CDMA according tothe prior art;

FIGS. 14A, 14B are diagrams useful in describing multiplexed signals ina complex plane according to the prior art;

FIG. 15 is a block diagram illustrating one channel of a CDMA receiversection in the CDMA receiver of a base station according to the priorart;

FIG. 16 is a diagram useful in describing interference cancellation;

FIG. 17 is a block diagram of an interference canceller;

FIG. 18 is a diagram showing the structure of an interferencecancellation unit according to the prior art;

FIG. 19 is a diagram useful in describing a spreading code according tothe prior art;

FIGS. 20A, 20B are diagrams useful in describing a channelidentification code according to the prior art;

FIG. 21 is a diagram useful in describing the relationship amongspreading factor, data and channel identification codes according to theprior art;

FIG. 22 is a block diagram of a receive demodulator according to theprior art;

FIG. 23 is a diagram showing the structure of a conventional despreadingunit in an interference cancellation unit; and

FIG. 24 is a diagram useful in describing the timing of a receive signaland interference replica according to the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(A) Principles

In general, the control channel DPCCH is such that the spreading factoris already known or fixed, while only the spreading factor of the datachannel DPDCH varies. The present invention is such that if thespreading factor SF on the data channel is not known, a channelidentification code is specified considering the spreading factor SF asbeing a minimum spreading factor SF_(min), and a code obtained bymultiplying this channel identification code and a user identificationcode together is adopted as the spreading code of the data channel.Despreading is performed using this spreading code, and an interferencereplica is produced. If this arrangement is adopted, the spreading codeof the data channel can be acquired immediately and it is unnecessary tospecify the spreading code upon obtaining spreading-factor informationby collecting one frame or one slot of control data (TFCI bits), as isrequired in the prior art. This makes it possible to reduce delay.

As mentioned above, a channel identification code having the minimumspreading factor can be used as the channel identification code. Thereason for this will now be described. In a case where there is only onedata channel, the channel identification code is C_(ch,SF,k) (wherek=SF/4), as described above in connection with FIG. 20. That is, thechannel identification code of a data channel for which minimumspreading factor SF_(min)=4 holds is (1, 1, −1, −1); the channelidentification code of a data channel for which minimum spreading factorSF_(min)=8 holds is (1, 1, −1, −1, 1, 1, −1, −1); and the channelidentification code of a data channel for which minimum spreading factorSF_(min)=16 holds is (1, 1, −1, −1, 1, 1, −1, −1, 1, 1, −1, −1, 1, 1,−1, −1). Accordingly, the relationship between the channelidentification code of the data channel having the spreading factor SFand the channel identification code of the data channel having theminimum spreading factor SF_(min) is such that the channelidentification code of the data channel having the spreading factor SFis the result of repeating, SF/SF_(min) times, the channelidentification code of the data channel having the minimum spreadingfactor SF_(min). Assume that the code of the minimum spreading factorSF_(min) is “0011”, where 1 and −1 have been restored to “0” and “1”,respectively. The code regarding the spreading factor SF will then be aniteration, SF/SF_(min) times, of the code “0011” of the spreading factorSF_(min) as follows: “00110011 . . . 0011”.

If the minimum spreading factor SF_(min) is considered to be thereference, one symbol will be spread by “0011” and therefore symbol datavaries every “0011”. In the case of the spreading factor SF, on theother hand, one symbol is spread by “00110011 . . . 0011”. Whendespreading is performed using the code “0011” of the minimum spreadingfactor SF_(min), therefore, the symbol data of the same code will bedemodulated SF/SF_(min) times.

Accordingly, user data of the data channel can be demodulated using thechannel identification code “0011” of the minimum spreading factor. Inthis case, the user data can be demodulated accurately if it is soarranged that the result of despreading is summed SF/SF_(min) times andthe “1”s and “0”s of the user data are demodulated by the result of thissumming operation.

The foregoing relates to a case where the number of data channels is oneand the code of the minimum spreading factor SF_(min) is “0011”. If thenumber of data channels is greater than one, however, the code of theminimum spreading factor SF_(min) will be any one of “0011”, “0101”,“0110”, as should be evident from FIG. 20. However, which code will beused is already known at the time of communication and therefore it willsuffice merely to use this code.

(B) First Embodiment

(a) Interference Cancellation Unit

FIG. 1 is a diagram showing the structure of an interferencecancellation unit according to a first embodiment of the presentinvention. Shown are a receiver 100, an interference cancellation unit200 according to this embodiment, and a receive demodulator 400. Theinterference cancellation unit 200 is provided for each user channelwithin the interference canceller (see FIG. 17); only one channel isshown in FIG. 1. The interference cancellation unit 200 includes adespreader 201 for multiplying a receive signal S by a despreading codethat is identical with the spreading code, thereby outputting a despreadsignal; a demodulator 202 for demodulating “1”, “0” of user data andcontrol data on the basis of the result of despreading; an attenuator203 for attenuating the demodulated signal by multiplying the result ofdemodulation by a damping coefficient that conforms to the degree ofreliability; a re-spreader 204 for spreading the demodulated signalagain to thereby output an interference replica; and a symbol-replicainterface 205 for creating and sending a symbol replica.

The despreader 201 has fingers 201 ₁ to 201 _(n). A searcher (not shown)detects multipath and inputs despread-start timing data and delay-timeadjustment data of each path to the fingers 201 ₁ to 201 _(n). Adespreading unit 201 a in each of the fingers 201 ₁ to 201 _(n) has aspreading-code generator 300 which generates (1) a spreading codeC_(DPCCH) for the control channel by multiplying a separately entereduser identification code SCi and a control-channel identification codeCCi together, and (2) a spreading code C_(DPDCH) for the data channel bymultiplying the user identification code SCi by a data-channelidentification code of a minimum spreading factor SF_(DPDCH) together.

A despreader 301 for the control channel despreads a direct wave ordelayed wave, which arrives via a prescribed path, based upon adespread-start timing from the searcher by multiplying the wave by thespreading code C_(DPCCH) for the control channel, and an integratingdelay unit 302 integrates the results of despreading and then applies adelay that conforms to the path and outputs a control-data signal of thecontrol channel DPCCH. A despreader 303 for the data channel similarlydespreads a direct wave or delayed wave, which arrives via a prescribedpath, by multiplying the wave by the spreading code C_(DPDCH) for thedata channel, and an integrating delay unit 304 integrates the resultsof despreading and then applies a delay that conforms to the path andoutputs a control-data signal of the data channel DPDCH.

A channel-estimation/AFC circuit 201 b estimates the fadingcharacteristic of the communication path using the pilot signalcontained in the despread control-data signal output from a selector 201g, and outputs a channel estimation signal. Channel compensation units201 c, 201 d multiply the despread control-data signal and despreaduser-data signal by the complex-conjugate signal of the channelestimation signal to thereby compensate for fading. RAKE combiners 201e, 201 f combine the despread signals (control-data signal and user-datasignal), from which fading has been eliminated, output from the fingersand output the results to demodulators 202 a, 202 b, respectively. Thedemodulators 202 a, 202 b discriminate “1”, “0” of the user data andcontrol data based upon the signals output from the RAKE combiners 201e, 201 f, respectively. Since the pilot signal is already known, aselector 203 a outputs the control data upon replacing the demodulatedpilot signal with the known pilot signal.

The attenuator 203 has multipliers 203 b, 203 c for multiplying thedemodulated user data and control data, respectively, by a first dampingcoefficient a that conforms to the degree of reliability, andmultipliers 203 d, 203 e for multiplying the user data and control data,respectively, by a second damping coefficient β that conforms to thedegree of reliability, thereby applying damping. The dampingcoefficients α, β are set in advance based upon transmission power, theinterference environment, etc., by way of example.

The symbol-replica interface 205 multiplies the output signals of themultipliers 203 b, 203 c by the channel estimation signal (complexsignal) that is output from the channel-estimation/AFC circuit 201 b,thereby adding on the fading characteristic of the transmission path,and sends the results of multiplication to the corresponding one of thereceive demodulators of the user channel as symbol-replica signals.

Multipliers 204 a, 204 b of each of the fingers 204 ₁ to 204 _(n) of there-spreader 204 multiply the user data and control data output from theattenuator 203 by the channel estimation signal (complex signal),thereby adding on the fading characteristic of the transmission path. Are-spread unit 204 c for the data channel spreads the user data, ontowhich fading had been added, by the spreading code C_(DPDCH) for thedata channel and outputs the spread signal. A re-spread unit 204 d forthe control channel spreads the user data, onto which fading had beenadded, by the spreading code C_(DPCCH) for the control channel andoutputs the spread signal. An adder 204 e combines the spread signals,which are output from the respective fingers, by data channel DPDCH andby control channel DPCCH, thereby generating an interference replica.This interference replica is combined with the interference replicasoutput from each of the interference cancellation units, the combinedsignal is subtracted from the receive signal and the resulting signal isinput to the receive demodulator of the corresponding user channel.

FIG. 2 is a diagram useful in describing the timings of a receive signaland interference replica according to the present invention, where SFirepresents the spreading factor. The spreading factor of the controlchannel DPCCH is SF1 and is fixed, whereas the spreading factor of thedata channel DPDCH takes on various values. In FIG. 2, the controlinformation is multiplexed in frame units. In the interference cancelleraccording to the present invention, the spreading factor is consideredto be the minimum spreading factor and the spreading code is generatedusing the code of the minimum spreading factor. As a result, it isunnecessary to find the spreading factor from the TFCI bits. Since it istherefore unnecessary to delay the receive signal in frame or slotunits, it is possible to realize a short delay, i.e., a delay on thesymbol order, so that a delay in control of transmission power can beeliminated.

(b) Receive Demodulator

A structure similar to that of the prior art shown in FIG. 22 can beadopted as the receive demodulator. Even use of a receive demodulator soconstructed will make it possible to eliminate delay in the loop oftransmission power control as a result of shortening delay in theinterference cancellation unit 200. With the arrangement of FIG. 22,however, it is necessary to specify the spreading code by obtaining thespreading factor from the TFCI bits in order to demodulate the userdata. The result is a delay in data demodulation.

FIG. 3 is a diagram showing the structure of a receive demodulator 400according to the present invention. The receive demodulator 400 includesfingers 401 ₁ to 401 _(n), RAKE combiners 402 a, 402 b of the data andcontrol channels, respectively, decoders 403 a, 403 b of the data andcontrol channels, respectively, a delay unit 404 for delaying the outputsignal of the RAKE combiner 402 a of the data channel until thespreading factor SF is found from the TFCI bits, and a despread-resultcombiner (second despreader) 405 for generating a despread signal of thereceive signal by integrating the results of despreading m(=SF/SF_(min)) times, where SF_(min) represents the minimum spreadingfactor.

Each of the fingers 401 ₁ to 401 _(n) has a despreader 501 the structureof which is identical with that of the despreader 201 a of theinterference cancellation unit (FIG. 1). More specifically, thedespreader of the control channel subjects the output signal of theinterference canceller to despread processing by multiplying this outputsignal by the spreading code C_(DPCCH) for the control channel,integrates the result of despreading, subsequently subjects theresulting signal to delay processing that conforms to the path andoutputs the processed signal. Further, the despreader of the datachannel subjects the output signal of the interference canceller todespread processing by multiplying this output signal by the spreadingcode C_(DPDCH) for the data channel, integrates the result ofdespreading, subsequently subjects the resulting signal to delayprocessing that conforms to the path and outputs the processed signal.In this case, the spreading code C_(DPDCH) for the data channel is theresult of multiplying the user identification code SCi by thedata-channel identification code regarding the minimum spreading factorSF_(min).

Combiners 502 a, 502 b generate transmit signals on the transmit side byadding the symbol replicas of the data and control channels DPDCH, DPCCHto the despread signals of the data and control channels DPDCH, DPCCH,respectively. A channel-estimation/AFC circuit 503 estimates the fadingcharacteristic of the communication path based upon a pilot signal thatenters from a selector 505, and channel correction units 504 a, 504 bapply channel correction processing to the signals output from thecombiners 502 a, 502 b, respectively, using the respective channelestimation signals, thereby eliminating fading.

The RAKE combiners 402 a, 402 b combine the data signals and controlsignals, respectively, output from the respective fingers and from whichfading has been eliminated, and output the combined. The delay unit 404delays the output signal of the RAKE combiner 402 a of the data channeluntil the spreading factor SF is found from the TFCI bits, and thedecoder 403 b applies error correction processing to the control-datasignal output from the RAKE combiner 402 b, decodes the control datathat prevailed prior to encoding and outputs the decoded data. Thedespread-result combiner 405 obtains the spreading factor SF from theTFCI bits contained in the control data, generates a despread signal ofthe receive signal by integrating the results of despreading m(=SF/SF_(min)) times, and inputs the generated despread signal to thedecoder 403 a. The latter decodes and outputs the user data from thedespread signal of the data channel.

In accordance with the receive decoder shown in FIG. 3, despreading ofthe data channel is begun immediately using the code conforming to theminimum spreading factor and the results of despreading are saved. Whenthe spreading factor SF is clarified, the saved results of despreadingare combined so that the user data of the data channel can bedemodulated with only a slight delay.

(C) Second Embodiment

FIG. 4 is a diagram showing the structure of an interferencecancellation unit according to a second embodiment of the presentinvention, in which components identical with those of the firstembodiment in FIG. 1 are designated by like reference characters. Thisembodiment differs from the first in that a data-disappearance detector206 for detecting disappearance of user data on the data channel isprovided and the value of the first damping coefficient a on the side ofthe data channel DPDCH is controlled based upon the absence or presenceof data.

In a packet mode, there are cases where no data is received solely onthe data channel DPDCH. In such case, the data-disappearance detector206 detects that user data is not being received and sets the value ofthe first damping coefficient α to “0” so that removal of interferencewill not be carried out. As a result, erroneous removal of interferencecan be prevented by not carrying out removal of interference in a slotor frame for which it has been determined that user data was notreceived on the data channel DPDCH.

Detection of data disappearance involves calculating power Pd of thedata channel and power Pc of the control channel in power calculationunits 206 a, 206 b, respectively, calculating a power ratio Pd/Pc by apower-ratio detector 206 c, comparing the power ratio Pd/Pc with a setvalue in terms of size in a comparator 206 d, deciding that user datahas disappeared if the power ratio Pd/Pc is less than the set value, anddeciding that user data exists if the power ratio Pd/Pc is equal to orgreater than the set value. A selector 206 e selects the first dampingcoefficient a and inputs it to the multiplier 203 b of the attenuator203 if user data exists, and selects 0 and inputs this to the multiplier203 b if user data does not exist.

The data-disappearance detector 206 of FIG. 4 is one example. Anarrangement can also be adopted in which filters (matched filters orlow-pass filters that pass only the symbol rates) corresponding to allspreading factors SF are provided and it is decided that user data hasdisappeared on the data channel DPDCH in a case where an output is notobtained from any filter whatsoever.

(D) Third Embodiment

FIG. 5 is a diagram showing the structure of an interferencecancellation unit according to a third embodiment of the presentinvention, in which components identical with those of the firstembodiment in FIG. 1 are designated by like reference characters.

There are instances where a signal on the data channel or controlchannel is transmitted upon being multiplied by the coefficient βc basedupon the magnitudes of the spreading factors of the data channel DPDCHand control channel DPCCH (see FIG. 13). In such case it is requiredthat the coefficient βc be reflected in the interference replica.Accordingly, in the third embodiment, a damping-coefficient decisionunit 207 is provided to discriminate the coefficient βc and cause thecoefficient βc to be reflected in the interference replica.

Discriminating the coefficient βc involves calculating the power Pd ofthe data channel and the power Pc of the control channel in the powercalculation units 207 a, 207 b, respectively, and calculating the powerratio Pd/Pc by a power-ratio detector 207 c. The power ratio is in 1:1correspondence with the ratio of the spreading factor of the datachannel to the spreading factor of the control channel. Accordingly, aβc discriminator 207 d obtains the βc from the power ratio and outputsthe βc to a multiplier 207 e, and the multiplier 207 e multiplies thecoefficient βc by the first damping factor α and outputs the product α′to the multiplier 203 b of the attenuator 203.

Thus, in accordance with the third embodiment, the damping coefficient αis corrected upon estimating βc based upon the receive power ratiobetween the data channel and control channel. As a result, it ispossible to create a more certain interference replica.

In this embodiment, averaging is not performed by a filter or the likein front of the βc discriminator or in front of the power-ratiodetector. However, a method of discriminating βc upon performingaveraging by a filter or the like can readily be devised.

(E) Fourth Embodiment

FIG. 6 is a diagram showing the structure of an interferencecancellation unit according to a fourth embodiment of the presentinvention, in which components identical with those of the firstembodiment in FIG. 1 are designated by like reference characters. Thefourth embodiment is a combination of the second and third embodimentsand is provided with a damping-coefficient decision unit 208.

The damping-coefficient decision unit 208 includes power calculationunits 208 a, 208 b for calculating the power Pd of the data channel andthe power Pc of the control channel, a power-ratio detector 208 c forcalculating the power ratio Pd/Pc, and a comparator 208 d for comparingthe power ratio Pd/Pc with a set value in terms of size, deciding thatuser data has disappeared if the power ratio Pd/Pc is less than the setvalue, and deciding that user data exists if the power ratio Pd/Pc isequal to or greater than the set value. A selector 208 e selects andoutputs the first damping coefficient α if user data exists, and selectsand outputs 0 if user data does not exist.

A βc discriminator 208 f obtains the coefficient βc from the power ratioand outputs βc, and a multiplier 208 g multiplies the coefficient βc bythe first damping factor α output from the selector 208 e and outputsthe product α′ to the multiplier 203 b of the attenuator 203.

In accordance with the fourth embodiment, erroneous removal ofinterference can be prevented by not carrying out removal ofinterference in a slot or frame for which it has been determined thatuser data was not received on the data channel DPDCH. Further, thedamping coefficient α is corrected upon estimating βc based upon thereceive power ratio between the data channel and control channel.

(F) Fifth Embodiment

FIG. 7 is a diagram showing the structure of an interferencecancellation unit according to a fifth embodiment of the presentinvention, in which components identical with those of the firstembodiment in FIG. 1 are designated by like reference characters. In thefirst embodiment, an interference replica is produced using a codeconforming to the minimum spreading factor SF_(min), In the fifthembodiment, however, a spreading-factor decision unit 209 is provided,the spreading factor SF of the data channel is estimated from the ratioof the power on the data channel DPDCH to the power on the controlchannel DPCCH, and the interference replica is produced using thisspreading factor.

The larger the spreading factor, the higher the process gain.Accordingly, in a W-CDMA system, transmission power of the controlchannel or data channel, whichever is larger, is reduced to lower thetotal transmission power. In other words, with W-CDMA, the control anddata channels are subjected to BPSK modulation to effect spread-spectrummodulation at powers that differ from each other, the spread-spectrummodulated signals are mapped on an I-Q complex plane and multiplexed andthe multiplexed signal is transmitted. Further, the spreading factor ofthe control channel is fixed. Thus, the spreading factor of the datachannel can be estimated from the ratio of the receive power of thecontrol channel to the receive power of the data channel.

The spreading-factor decision unit 209 has power calculation units 209a, 209 b for calculating the power Pd of the data channel and the powerPc of the control channel, a power-ratio detector 209 c for calculatingthe power ratio Pd/Pc, and an SF discriminator 209 d for estimating thespreading factor SF from the power ratio Pd/Pc and outputting thespreading factor SF. It should be noted that a correspondence table ofcorresponding power ratios Pd/Pc and spreading factors SF can beprovided beforehand and the spreading factor SF can be obtained from thetable.

A despread-result combiner (second despreader) 210 calculatesm=SF/SF _(min)using the minimum spreading factor SF_(min) and the estimated spreadingfactor SF, and integrates the results of despreading m times to generatethe despread signal of the receive signal. The demodulator 202 a for thedata channel demodulates the user data from the despread signal that isoutput from the despread-result combiner 210.

In accordance with the fifth embodiment, despreading is performed uponestimating the spreading factor SF of the data channel. This makes itpossible to produce a precise interference replica.

(G) Sixth Embodiment

FIG. 8 is a diagram showing the structure of an interferencecancellation unit according to a sixth embodiment of the presentinvention. This is a combination of the second and fifth embodiments, inwhich components identical with those of these embodiments aredesignated by like reference characters.

In accordance with the sixth embodiment, erroneous removal ofinterference can be prevented by not carrying out removal ofinterference in a slot or frame for which it has been determined thatuser data was not received on the data channel DPDCH. Further, inaccordance with the sixth embodiment, despreading is performed uponestimating the spreading factor SF of the data channel. This makes itpossible to produce a precise interference replica.

(H) Seventh Embodiment

FIG. 9 is a diagram showing the structure of an interferencecancellation unit according to a seventh embodiment of the presentinvention. This is a combination of the third and fifth embodiments, inwhich components identical with those of these embodiments aredesignated by like reference characters. It should be noted that thedamping-coefficient decision unit 207 can be replaced by thedamping-coefficient decision unit 208 of the fourth embodiment.

In accordance with the seventh embodiment, the damping coefficient α iscorrected upon estimating βc based upon the ratio of the receive poweron the data channel to the receive power on the control channel. As aresult, a more reliable interference replica can be created. Further, inaccordance with the seventh embodiment, despreading is performed uponestimating the spreading factor SF of the data channel. This makes itpossible to produce a precise interference replica.

Thus, in accordance with the present invention, as described above, itis so arranged that a first code is regarded as a code decided basedupon a minimum spreading factor, and despreading necessary forinterference cancellation is carried out using a despreading codeobtained by combining the first code and a second code that differs forevery user. As a result, processing for despreading/re-spreading can beexecuted immediately without the need to decide a spreading code after aspreading factor is discriminated from control information (TFCI bits).This makes it possible to shorten delay time till generation of theinterference replica and to reduce delay in the loop of transmissionpower control.

In accordance with the present invention, the fact that user data is notbeing transmitted on the data channel DPDCH is detected and, inresponse, the damping coefficient of the data channel DPDCH is madezero. When data on the data channel DPDCH is not being received,therefore, interference cancellation is not carried out. This makes itpossible to prevent erroneous interference cancellation.

In accordance with the present invention, the damping coefficient of thedata channel is changed based upon the ratio of receive power on thedata channel to receive power on the control channel. As a result, evenin a case where a coefficient βc that is based upon the spreading factorof the data channel DPDCH and the spreading factor of the controlchannel DPCCH on the transmit side is used to multiply the signal onwhichever of the channels has the larger spreading factor and theresulting signal is transmitted, the coefficient βc can be reflected inthe interference replica. This makes it possible to produce a highlyprecise interference replica.

In accordance with the present invention, it is so arranged that ahighly precise interference replica is generated by estimating thespreading factor SF from the power ratio between the data channel DPDCHand the control channel DPCCH, and producing the interference replicausing this estimated spreading factor, which has a higher degree ofcertainty.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the appended claims.

1. A spread-spectrum signal receiver apparatus for receiving aspread-spectrum signal and demodulating transmit data from the signal,comprising: a receive unit for receiving a spread-spectrum signal thathas been spread by a spreading code comprising a combination of a firstcode that varies depending upon a spreading factor and a second codethat differs for every user; an interference canceller for producing areplica of an interference signal from the receive signal using adespreading code comprising a combination of the first code, which isregarded as a code decided based upon a minimum spreading factor, andthe second code that differs for every user, and generating a signalobtained by subtracting the replica from the receive signal; and ademodulator for demodulating transmit data, from the signal from whichthe replica has been subtracted, by despread processing using aspreading code on the transmit side.
 2. The apparatus according to claim1, wherein said interference canceller includes: a despreader fordespreading the receive signal using a despreading code comprising acombination of at least the first code decided based upon the minimumspreading factor and the second code that differs for every user; ademodulator for demodulating transmit data from the despread signal; anattenuator for multiplying the demodulated transmit data by a prescribeddamping coefficient; and a spreader for generating the replica byspreading the attenuated transmit data using a code identical with thedespreading code.
 3. The apparatus according to claim 1, wherein thefirst code decided by the spreading factor is obtained by systematicallyvarying a code that conforms to the minimum spreading factor.
 4. Aninterference cancellation apparatus for receiving a spread-spectrumsignal that has been spread by a spreading code comprising a combinationof a first code that varies depending upon spreading factor and a secondcode that differs for every user, and generating a replica of aninterference signal from the receive signal, comprising: a receiver forreceiving the spread-spectrum signal; and a replica producing unit forproducing a replica of the interference signal from the receive signalusing a despreading code comprising a combination of the first code,which is regarded as a code decided based upon a minimum spreadingfactor, and the second code that differs for every user.
 5. Theapparatus according to claim 4, wherein said replica producing unitincludes: a despreader for despreading the receive signal using thedespreading code comprising a combination of the first code and thesecond code that differs for every user; a demodulator for demodulatingtransmit data from the despread signal; an attenuator for multiplyingthe demodulated transmit data by a prescribed damping coefficient; aspreader for generating the replica by spreading the attenuated transmitdata using a code identical with the despreading code.
 6. The apparatusaccording to claim 5, further comprising a damping-coefficient alteringunit for setting the damping coefficient to zero upon detecting thatdata is not being transmitted.
 7. The apparatus according to claim 5,further comprising a damping-coefficient altering unit for altering adamping coefficient of a data channel based upon the ratio ofreceive-signal power of the data channel to receive-signal power of acontrol channel, wherein the data and control channels are included inthe receive signal.
 8. The apparatus according to claim 4, wherein saidreplica producing unit includes: a first despreader for despreading areceive signal using the despreading code comprising the combination ofthe first code, which is regarded as a code decided based upon a minimumspreading factor, and the second code that differs for every user; aspreading-factor estimation unit for estimating the spreading factor(SF) on the transmit side; a second despreader for generating a despreadsignal of the receive signal by integrating, m times, the result ofdespreading, which is output from said first despreader, based upon thedespreading code conforming to the minimum spreading factor, where m (aninteger) represents the ratio of the estimated spreading factor to theminimum spreading factor; a demodulator for demodulating the transmitdata from the despread signal; an attenuator for multiplying thedemodulated transmit data by a prescribed damping coefficient; and aspreader for generating the replica by spreading the attenuated transmitdata using a code identical with the despreading code.
 9. The apparatusaccording to claim 8, wherein said spreading-factor estimation unitestimates the spreading factor based upon the ratio of receive-signalpower of a data channel to receive-signal power of a control channel,wherein the data and control channels are included in the receivesignal.
 10. The apparatus according to claim 8, further comprising adamping-coefficient altering unit for setting the damping coefficient ofa data channel to zero upon detecting that data is not being transmittedon the data channel.
 11. The apparatus according to claim 8, furthercomprising a damping-coefficient altering unit for altering a dampingcoefficient of a data channel based upon the ratio of receive-signalpower of a data channel to receive-signal power of a control channel.